The requirement for data collection usually involves the operation of receiving analog data, converting that analog data to a digital format, and then processing the data in the digital format for conversion thereof or storage thereof. A class of devices has been developed for such applications that utilize mixed-signal technology to provide a “system-on-a-chip” (SOC) solution. These SOCs, sometimes referred to as microcontroller units (MCUs), are comprised of a digital section and may include an analog section. The analog section typically includes an analog-to-digital converter that may have the input thereof multiplexed such that it can sample analog values from a plurality of different sources, such as various sensors or transducers.
These MCUs are designed to operate on very low power, such that they can be disposed in remote areas and then run off of battery power for long durations of time. Since sampling of data is a rather intermittent operation, it is advantageous for these devices to have the ability to enter into a low power operating mode. An 80 kHz low frequency oscillator is provided on-chip for low power operation. The power budget for these types of MCU chips is divided among a number of functional elements. On the digital side, the processor has a large number of gates and typically requires a relatively high frequency of operation to provide adequate capabilities, on the order of 25 MHz. There also a plurality of timers, Input/Output (I/O) devices, etc., but the largest portion of the power budget is the processor. The clock circuitry can also take up a fair portion of the power budget, even when not driving the processor, but the primary portion of the power budget due to the power requirement of the digital processing and the clock speed thereof.
These MCUs have clock speeds ranging from 25 MHz to 100 MHz. Even if the processor is not processing information, the clock is still clocking the chip at a relatively high frequency and, as long as gates are being driven to different states, power will be dissipated. Therefore, most of these MCU devices have a low power operating mode which allows the processor or a digital portion thereof to be placed into a very “deep sleep” mode by halting the processing operation or just allow a lower power mode. In the lower power mode, it is sometimes necessary to lower the clock speed down to a low clock operating speed of, for example, 32 kHz. This can allow the processor to continue operating, but at a much lower speed.
Further, another source of power draw can be the data converters, analog-to-digital converters (ADCS) and the digital-to-analog converters (DACS) which can be disposed in an analog section. Even though this portion of the chip may operate at a lower clock speed, the time to sample an input at this clock speed might not be acceptable. Therefore, the processor will typically run at the lower clock speed for background processing operations and would be raised to the higher clock speed when doing a sampling operation. Multiple components or functional blocks associated with the MCU could be turned off, depending upon the power management philosophy upon which the part is based. However, the lowest power operation is to place the digital portion in a deep sleep mode wherein the processing operation is substantially terminated, with the configuration information for the “current state” of the processor maintained. An external monitoring circuit will monitor some conditions such as an interrupt generated by another block, an external event, etc., and will then wake up the part and initiate processing at the last current state.
In some applications, a real time clock function is required. This is easily facilitated in the MCU running at the high speed, since there are typically provided timers and the such on-board that can be clocked by a high frequency clock. One type of MCU that provides for this is the family of MCU products, C8051FXXX, manufactured by Silicon Laboratories Inc. However, to provide the real time clock functionality, the count must be segmented, or perceived in some way such that it can count seconds, minutes, hours, days, etc. and stored in a register. Typically, the entire real time clock function could be carried out in the background with a low frequency clock. In some of these MCU devices, there is provided a high frequency clock for operating the processor in the default operating mode which is the high power operating mode and a low frequency clock for operating the processor in the low power mode.
A low frequency clock can be utilized to clock the timers independent of the operation of the processor and to generate interrupts. However, the functionality of the real time clock basically takes advantage of the operation of the entire digital section. As such, in very low power applications wherein it is desirable to have time-stamp information on samples that are taken, the real time clock must be run during the very low power mode. Therefore, it would be desirable to have none of the digital section operating between taking samples. Additionally, it would be beneficial if the circuit could operate in the low power applications using an external clock signal without the use of any internal clock to completely eliminate the power requirements of the internal clock circuits while being able to communicate with the MCU reliably when needed.